The invention relates generally to transcutaneous prosthetic devices. More specifically, the invention relates to an interface between a transcutaneous prosthesis and the skin of a patient.
Approximately two million persons were living with limb loss in the United States in 2007. The main causes of limb loss are vascular disease (54%), including diabetic vasculopathy and peripheral arterial disease, trauma (45%), and cancer (less than 2%). As a result, approximately 185,000 amputations occur in the United States each year.
Limb prostheses, which are used to recover some functionality, are typically mated to the residual stump (residuum) of amputated limbs using custom conformal sockets. Socket attachment can be achieved by creating a vacuum between the residuum and the prosthesis. As the patient dons the prosthesis, air is expelled from the socket through a one-way valve. The negative pressure around the residuum holds the prosthesis in place until the user releases it by opening the valve. The socket attachment method is not an ideal solution. Problems include: phantom pain due to loss of osseoperception; difficulty in properly attaching the prosthesis from changes in skin condition and/or residuum volume; difficulty fitting short residuums; skin irritation; lack of robust stabilization between the prosthesis and residual limb; and, in general, difficulties from frequently donning and doffing the socket.
Direct skeletal attachment (DSA) is an alternative method of prosthesis attachment that can provide osseoperception, improved locomotor activities of a patient, and elimination of other problems associated with donning and using a socket. In the DSA approach an intramedullary stem integrates with intact bone, and a percutaneous pylon attached to the stem acts as a mounting post for the prosthesis. See, for example, U.S. Pat. No. 3,947,897, which describes an apparatus for connecting a prosthesis to a bone of a residuum.
Because the DSA implant protrudes through the skin of the patient, DSA implants are susceptible to infections. To address this issue, DSA implants incorporate skin-to-DSA interfaces comprising various porous architectures for skin ingrowth and implant cutaneous integration as barriers against pathogens traveling down the pylon down to the stem and the surrounding tissues, in particular bone. However, current skin-to-DSA interfaces often fail, leading to infection and implant instability, requiring DSA device removal and replacement either with another DSA implant or more conventional socket suspension system.
Skin-to-DSA interface failures can occur due to the mismatch in mechanical compliance between pliable skin and the more rigid DSA interface or the DSA device itself, which are often composed of titanium alloys such as Ti-6AI-4V. This mismatch can lead to stress risers that cause the skin to tear away from the interface as the skin moves relative to the bone during normal motion or as the recipient gains or losses weight. To minimize tearing, it is thought that the mobility of skin around the implant should be minimized; both surgical techniques and devices for this purpose have been developed.
Devices attempting to solve this problem include a percutaneous bar with a flexible mesh collar, holes at the subcutaneous perimeter of a flange, and a collar made of a stainless steel spring or nylon hooks. Animal studies with these devices produced promising results, however, many of the implants are sensitive to its positioning relative to the dermal and subcutaneous tissues and do not tolerate junction shifting when the distance from the bone to the skin-binding junction changes. Another approach was positioning of a bar with a porous flange in the dermal tissues immediately below the epithelium. While this may reduce the mobility of skin in the plane parallel to the flange, the attachment to the solid bar still remained fragile. In another device, an interface design provides a dome-shaped device with holes for skin attachment; however, the interface is rigid and therefore does not address the problem of compliance mismatch.
When the skin at the skin-to-DSA interface tears, it creates entry points for bacteria and other pathogens into the body. Tears can self-repair by reepithelization, but the repairs are weaker after each tear. For example, recurring atrophic or hyper-trophic scarring and callus formation at the skin-to-implant interface will incrementally reduce the strength of the tissue adhesion in subsequent repairs, thus spiraling into weaker dermal and epidermal integration and thereby increase the risk of further tears and infection. While the initial clinical studies using DSA limb prostheses in humans were conducted in the U.S. in the mid 1970's, the FDA does not currently allow DSA procedures, in part because of a lack of compelling evidence for a solution to the skin seal problem.
Despite these problems, DSA prosthetic devices are permitted in other countries. Over 150 patients in Sweden, Germany, the Netherlands, and Australia have received DSA devices, and analysis and in-depth interviews with patients living with osseointegrated prostheses objectively confirmed functional improvements. Participants described their experience with DSA prostheses as making a revolutionary change to the quality of their lives. However, improved DSA interfaces are still required to minimize infection and reduce the need or surgical removal or periodic replacement of the DSA device. It would therefore be advantageous to develop a DSA interface that reduces skin tearing.